This invention relates to a high-voltage stabilizing power supply apparatus with an improved inductive noise compensation and, more particularly, to a high-voltage stabilizing power supply apparatus suitable for supplying an anode voltage of a cathode ray tube (CRT).
Generally, an anode of a CRT used in a display, such as a television receiver, receives a high voltage DC output from a kind of DC-DC converter wherein horizontal flyback pulses are boosted by a flyback transformer and are rectified and smoothed by a rectifier-smoothing circuit. A stabilizing power supply is used, in this case, that has a technique to improve the high-voltage stability of the CRT against a change in anode current (generally a load current).
In a conventional apparatus as such a high-voltage stabilizing power supply, a high-resistance resistor voltage divider is provided between a pair of output terminals in order to detect a variation in output voltage corresponding to the variation in load current. A pulse width modulation (PWM) control signal generator generates a PWM control signal having a pulse width in accordance with the amplitude of an error voltage corresponding to the variation in output voltage detected by a low-voltage-side resistor of the resistor voltage divider. A switching element switches a DC voltage input supplied to the primary winding of a step-up transformer based on the PWM control signal. The rectifier-smoothing circuit rectifies and smoothes the output from the secondary winding of the step-up transformer and supplies a stabilizing high-voltage DC output to the pair of output terminals.
In the high-voltage stabilizing power supply as described above, external inductive noise or a switching frequency (converter frequency) component of about, e.g., 10 to 100 kHz, induced in the resistor voltage divider causes a problem. If such inductive noise exists, variation detection of the high-voltage output by the resistor voltage divider is interfered with, and high-voltage stability cannot be obtained.
In order to solve the above problem, a single shield for induction prevention is provided to the resistor voltage divider, or a capacitor for compensating the inductive noise is connected in parallel with the low-voltage side of the resistor voltage divider.
However, when such countermeasures are taken, the frequency response characteristics are degraded by the influence of the stray capacitance at the node between the resistor voltage divider and the shield. This is because, for example, a rectangular wave high-voltage output variation has an unpreferable transient (a so-called undercut characteristic) wherein the leading and trailing edges of a waveform detected by the low-voltage-side resistor of the resistor voltage divider are rounded. This means that the high-voltage stabilizing power supply has poor response characteristics against a dynamic variation in load current.
In order to compensate for the degradation in the response characteristics, it is proposed to connect another capacitor in parallel with the high-voltage-side resistor of the terminal voltage divider. However, with this countermeasure, since the capacitor to be connected to the high-voltage-side resistor must have a considerably high withstand voltage property, the size and cost of the overall apparatus are increased and reliability suffers.